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 * Copyright (c) 2012, 2013, Oracle and/or its affiliates. All rights reserved.
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
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package java.util;

import java.util.function.Consumer;
import java.util.function.Function;
import java.util.function.Predicate;
import java.util.function.Supplier;

/**
 * A container object which may or may not contain a non-null value.
 * If a value is present, {@code isPresent()} will return {@code true} and
 * {@code get()} will return the value.
 *
 * <p>Additional methods that depend on the presence or absence of a contained
 * value are provided, such as {@link #orElse(java.lang.Object) orElse()}
 * (return a default value if value not present) and
 * {@link #ifPresent(java.util.function.Consumer) ifPresent()} (execute a block
 * of code if the value is present).
 *
 * <p>This is a <a href="../lang/doc-files/ValueBased.html">value-based</a>
 * class; use of identity-sensitive operations (including reference equality
 * ({@code ==}), identity hash code, or synchronization) on instances of
 * {@code Optional} may have unpredictable results and should be avoided.
 *
 * @since 1.8
 */
public final class Optional<T> {

  /**
   * Common instance for {@code empty()}.
   */
  private static final Optional<?> EMPTY = new Optional<>();

  /**
   * If non-null, the value; if null, indicates no value is present
   */
  private final T value;

  /**
   * Constructs an empty instance.
   *
   * @implNote Generally only one empty instance, {@link Optional#EMPTY}, should exist per VM.
   */
  private Optional() {
    this.value = null;
  }

  /**
   * Returns an empty {@code Optional} instance.  No value is present for this
   * Optional.
   *
   * @param <T> Type of the non-existent value
   * @return an empty {@code Optional}
   * @apiNote Though it may be tempting to do so, avoid testing if an object is empty by comparing
   * with {@code ==} against instances returned by {@code Option.empty()}. There is no guarantee
   * that it is a singleton. Instead, use {@link #isPresent()}.
   */
  public static <T> Optional<T> empty() {
    @SuppressWarnings("unchecked")
    Optional<T> t = (Optional<T>) EMPTY;
    return t;
  }

  /**
   * Constructs an instance with the value present.
   *
   * @param value the non-null value to be present
   * @throws NullPointerException if value is null
   */
  private Optional(T value) {
    this.value = Objects.requireNonNull(value);
  }

  /**
   * Returns an {@code Optional} with the specified present non-null value.
   *
   * @param <T> the class of the value
   * @param value the value to be present, which must be non-null
   * @return an {@code Optional} with the value present
   * @throws NullPointerException if value is null
   */
  public static <T> Optional<T> of(T value) {
    return new Optional<>(value);
  }

  /**
   * Returns an {@code Optional} describing the specified value, if non-null,
   * otherwise returns an empty {@code Optional}.
   *
   * @param <T> the class of the value
   * @param value the possibly-null value to describe
   * @return an {@code Optional} with a present value if the specified value is non-null, otherwise
   * an empty {@code Optional}
   */
  public static <T> Optional<T> ofNullable(T value) {
    return value == null ? empty() : of(value);
  }

  /**
   * If a value is present in this {@code Optional}, returns the value,
   * otherwise throws {@code NoSuchElementException}.
   *
   * @return the non-null value held by this {@code Optional}
   * @throws NoSuchElementException if there is no value present
   * @see Optional#isPresent()
   */
  public T get() {
    if (value == null) {
      throw new NoSuchElementException("No value present");
    }
    return value;
  }

  /**
   * Return {@code true} if there is a value present, otherwise {@code false}.
   *
   * @return {@code true} if there is a value present, otherwise {@code false}
   */
  public boolean isPresent() {
    return value != null;
  }

  /**
   * If a value is present, invoke the specified consumer with the value,
   * otherwise do nothing.
   *
   * @param consumer block to be executed if a value is present
   * @throws NullPointerException if value is present and {@code consumer} is null
   */
  public void ifPresent(Consumer<? super T> consumer) {
    if (value != null) {
      consumer.accept(value);
    }
  }

  /**
   * If a value is present, and the value matches the given predicate,
   * return an {@code Optional} describing the value, otherwise return an
   * empty {@code Optional}.
   *
   * @param predicate a predicate to apply to the value, if present
   * @return an {@code Optional} describing the value of this {@code Optional} if a value is present
   * and the value matches the given predicate, otherwise an empty {@code Optional}
   * @throws NullPointerException if the predicate is null
   */
  public Optional<T> filter(Predicate<? super T> predicate) {
    Objects.requireNonNull(predicate);
    if (!isPresent()) {
      return this;
    } else {
      return predicate.test(value) ? this : empty();
    }
  }

  /**
   * If a value is present, apply the provided mapping function to it,
   * and if the result is non-null, return an {@code Optional} describing the
   * result.  Otherwise return an empty {@code Optional}.
   *
   * @param <U> The type of the result of the mapping function
   * @param mapper a mapping function to apply to the value, if present
   * @return an {@code Optional} describing the result of applying a mapping function to the value
   * of this {@code Optional}, if a value is present, otherwise an empty {@code Optional}
   * @throws NullPointerException if the mapping function is null
   * @apiNote This method supports post-processing on optional values, without the need to
   * explicitly check for a return status.  For example, the following code traverses a stream of
   * file names, selects one that has not yet been processed, and then opens that file, returning an
   * {@code Optional<FileInputStream>}:
   *
   * <pre>{@code
   *     Optional<FileInputStream> fis =
   *         names.stream().filter(name -> !isProcessedYet(name))
   *                       .findFirst()
   *                       .map(name -> new FileInputStream(name));
   * }</pre>
   *
   * Here, {@code findFirst} returns an {@code Optional<String>}, and then {@code map} returns an
   * {@code Optional<FileInputStream>} for the desired file if one exists.
   */
  public <U> Optional<U> map(Function<? super T, ? extends U> mapper) {
    Objects.requireNonNull(mapper);
    if (!isPresent()) {
      return empty();
    } else {
      return Optional.ofNullable(mapper.apply(value));
    }
  }

  /**
   * If a value is present, apply the provided {@code Optional}-bearing
   * mapping function to it, return that result, otherwise return an empty
   * {@code Optional}.  This method is similar to {@link #map(Function)},
   * but the provided mapper is one whose result is already an {@code Optional},
   * and if invoked, {@code flatMap} does not wrap it with an additional
   * {@code Optional}.
   *
   * @param <U> The type parameter to the {@code Optional} returned by
   * @param mapper a mapping function to apply to the value, if present the mapping function
   * @return the result of applying an {@code Optional}-bearing mapping function to the value of
   * this {@code Optional}, if a value is present, otherwise an empty {@code Optional}
   * @throws NullPointerException if the mapping function is null or returns a null result
   */
  public <U> Optional<U> flatMap(Function<? super T, Optional<U>> mapper) {
    Objects.requireNonNull(mapper);
    if (!isPresent()) {
      return empty();
    } else {
      return Objects.requireNonNull(mapper.apply(value));
    }
  }

  /**
   * Return the value if present, otherwise return {@code other}.
   *
   * @param other the value to be returned if there is no value present, may be null
   * @return the value, if present, otherwise {@code other}
   */
  public T orElse(T other) {
    return value != null ? value : other;
  }

  /**
   * Return the value if present, otherwise invoke {@code other} and return
   * the result of that invocation.
   *
   * @param other a {@code Supplier} whose result is returned if no value is present
   * @return the value if present otherwise the result of {@code other.get()}
   * @throws NullPointerException if value is not present and {@code other} is null
   */
  public T orElseGet(Supplier<? extends T> other) {
    return value != null ? value : other.get();
  }

  /**
   * Return the contained value, if present, otherwise throw an exception
   * to be created by the provided supplier.
   *
   * @param <X> Type of the exception to be thrown
   * @param exceptionSupplier The supplier which will return the exception to be thrown
   * @return the present value
   * @throws X if there is no value present
   * @throws NullPointerException if no value is present and {@code exceptionSupplier} is null
   * @apiNote A method reference to the exception constructor with an empty argument list can be
   * used as the supplier. For example, {@code IllegalStateException::new}
   */
  public <X extends Throwable> T orElseThrow(Supplier<? extends X> exceptionSupplier) throws X {
    if (value != null) {
      return value;
    } else {
      throw exceptionSupplier.get();
    }
  }

  /**
   * Indicates whether some other object is "equal to" this Optional. The
   * other object is considered equal if:
   * <ul>
   * <li>it is also an {@code Optional} and;
   * <li>both instances have no value present or;
   * <li>the present values are "equal to" each other via {@code equals()}.
   * </ul>
   *
   * @param obj an object to be tested for equality
   * @return {code true} if the other object is "equal to" this object otherwise {@code false}
   */
  @Override
  public boolean equals(Object obj) {
    if (this == obj) {
      return true;
    }

    if (!(obj instanceof Optional)) {
      return false;
    }

    Optional<?> other = (Optional<?>) obj;
    return Objects.equals(value, other.value);
  }

  /**
   * Returns the hash code value of the present value, if any, or 0 (zero) if
   * no value is present.
   *
   * @return hash code value of the present value or 0 if no value is present
   */
  @Override
  public int hashCode() {
    return Objects.hashCode(value);
  }

  /**
   * Returns a non-empty string representation of this Optional suitable for
   * debugging. The exact presentation format is unspecified and may vary
   * between implementations and versions.
   *
   * @return the string representation of this instance
   * @implSpec If a value is present the result must include its string representation in the
   * result. Empty and present Optionals must be unambiguously differentiable.
   */
  @Override
  public String toString() {
    return value != null
        ? String.format("Optional[%s]", value)
        : "Optional.empty";
  }
}
